How many encoders are needed to control an H/slide drive?

[pipsqueaker]

We're planning on going an H drive this year, and our coder says that he wants five encoders, one on each wheel.

My interpretation of H-drive is basically that it is a tank drive with an extra wheel in the middle. This should mean that both wheels on the left and right are being driven in the same direction at the same rate- because of this I think that one encoder per side is fine, as the wheels will be driven with the same motors and gearboxes in an identical manner- our coder says that, because the two wheels on a side wont be driven by the same motor/gearbox, we will need to place an encoder on each wheel to correct for any tiny variations.

To me this seems excessive and inconvenient (5 encoders will take up all our DIO ports on the roborio, requiring me to expand it). I just wanted your opinions on whether we truly need an encoder on every single wheel.

EDIT: I should clarify, the encoders are for autonomous stuff, we're not really planning on using them in teleop

[Chris Hibner]

When we did ours in 2011, we used three encoders. It worked well.

[ASmith1675]

5 encoders certainly aren't NEEDED. 4 on the main drive will allow programmers to be able to speed match the 4 wheel, which will allow the best possible "drive straight" functionality in autonomous. You can get away with 2, but you won't be able to guarantee all 4 main drive motors are turning at the same rate then.

The 5th motor encoder would only be needed if you planned to do any strafing in autonomous.

[RonnieS]

When we did ours in 2011, we used three encoders. It worked well.

2011 was flat, how do you feel about using three when trans-versing the bump, slippage on some wheels and not others affect anything?
DISCLAIMER: I am not a programmer, just interested.

[nighterfighter]

You would want 5 encoders, if you were powering each wheel separately. But if you aren't going to power each wheel separately, that is a non-issue. I'd imagine you would want 3 total, for drive. One for the left side, with the encoder attached to the gearbox output shaft (Well, the little doohickey that sticks out for encoders), same thing for the right side, then one encoder on your center motor (for the H drive). (Not necessarily on the motor itself, of course. But I'm sure you know where to put it.)

Even if you wanted to use more than 5 or 6 encoders, without expanding the DIO ports on the roboRIO, the Jaguars DO have a built-in interface for encoders, that are accessible via CAN. (The Talons might have them also, you'll have to look that up).

[Kevin Sevcik]

Even if you wanted to use more than 5 or 6 encoders, without expanding the DIO ports on the roboRIO, the Jaguars DO have a built-in interface for encoders, that are accessible via CAN. (The Talons might have them also, you'll have to look that up).Talons. Talons, Talons, Talons. The encoder input on the Jags is nearly useless. It won't give you feedback unless you're actually using a closed-loop control mode. And its closed loop controls aren't all that great. And you can only use one encoder and one Jag to drive one motor, unless you're doing something to split the encoder signal, because there's no way to slave on Jag to another. Talons on the other hand give you a constant feedback stream of both velocity and position, so you could put the PID on your roboRIO, or use the PID on the Talon. And if you're in velocity mode, you can still get position feedback for all your autonomous needs.

[nighterfighter]

Talons. Talons, Talons, Talons. The encoder input on the Jags is nearly useless. It won't give you feedback unless you're actually using a closed-loop control mode. And its closed loop controls aren't all that great. And you can only use one encoder and one Jag to drive one motor, unless you're doing something to split the encoder signal, because there's no way to slave on Jag to another. Talons on the other hand give you a constant feedback stream of both velocity and position, so you could put the PID on your roboRIO, or use the PID on the Talon. And if you're in velocity mode, you can still get position feedback for all your autonomous needs.

Okay cool! Like I said, I wasn't sure about the capabilities of Talons.

As for the closed loop part of the Jaguars, I suppose that's true. I've only ever used them with closed loop, I never anything else.

[Kevin Sevcik]

Okay cool! Like I said, I wasn't sure about the capabilities of Talons.

As for the closed loop part of the Jaguars, I suppose that's true. I've only ever used them with closed loop, I never anything else.Actually I should amend that to Talon SRX. The original Talon and Talon SR controllers aren't CAN capable, after all.

[Bruceb]

wont that center wheel on the H drive cause a problem going over the bump?

[Jarren Harkema]

wont that center wheel on the H drive cause a problem going over the bump?

Only if it is in a fixed in position. Or... Are there other ways to obtain H drive motion with a different position/number of omni wheels?

[jeremylee]

The encoder input on the Jags is nearly useless. It won't give you feedback unless you're actually using a closed-loop control mode.
1736 used Jags via CAN in 2012 and we were able to read speed while in open loop to implement PID on the cRIO. From BDC comm, I remember having to switch to closed loop, configure the encoder, and switch back to open loop to see speed. This wasn't an issue on the robot though.

[Ether]

Are there other ways to obtain H drive motion with a different position/number of omni wheels?

Yes.

http://www.chiefdelphi.com/forums/showpost.php?p=1419374&postcount=11

Also try searching for the words "Killough" and "Kiwi" here on CD

[Chris Hibner]

2011 was flat, how do you feel about using three when trans-versing the bump, slippage on some wheels and not others affect anything?
DISCLAIMER: I am not a programmer, just interested.

We put the encoders on non-driven follower wheels that year. They were Vex omni-wheels mounted to tiny swing arms spring loaded to contact the ground. Therefore, no real worries about wheel slip.

[Jarren Harkema]

Yes.

http://www.chiefdelphi.com/forums/showpost.php?p=1419374&postcount=11

Also try searching for the words "Killough" and "Kiwi" here on CD




Thanks Ether, though it was more of a question for BruceB and Pipsqueeker. Our team thinks we have figured out a way to use all omni and get over the scoring platform. :)